Abnormality diagnosis system and method for diagnosing abnormality in filter regeneration system

ABSTRACT

The invention intends to provide a technology which makes it possible to diagnose with higher accuracy whether or not abnormality occurs in a filter regeneration system causing excessive execution frequency of a regeneration process. The filter regeneration system initiates execution of the regeneration process, incase an estimated particulate matter (PM) accumulation amount at the filter reaches a pre-determined regeneration requiring accumulation amount; or in case the pressure upstream of the filter or the differential pressure across the filter reaches a pre-determined regeneration requiring value, the value being larger than the pressure or the differential pressure corresponding to the regeneration requiring accumulation amount. Then, the diagnosis is carried out based on a ratio of an estimated PM accumulation amount at the initiation of the execution of the regeneration process to the regeneration requiring accumulation amount.

TECHNICAL FIELD

The present invention relates to a system and a method for diagnosingabnormality in a system executing a regeneration process for aparticulate filter placed in an exhaust passage of an internalcombustion engine.

BACKGROUND ART

A particulate filter (hereinafter simply referred to as a “filter”) maybe placed in an exhaust passage of an internal combustion engine to trapparticulate matter (hereinafter referred to as “PM”) in the exhaust gasof the internal combustion engine. In this case, operation of theinternal combustion engine may be negatively affected by increase of theback-pressure, if the PM accumulation amount at the filter shouldincrease excessively. Further, if the PM accumulates excessively at thefilter and is oxidized, the filter temperature may increase excessivelyby the oxidation heat, which may eventually cause erosion or breakage ofthe filter. To prevent such problems, a filter regeneration systemconducting a regeneration process by oxidizing and removing the PMaccumulated at the filter has been employed.

Patent Document 1 describes a technology concerning an exhaust gaspurification device carrying out a regeneration process as describedabove. The exhaust gas purification device disclosed in Patent Document1 is provided with the first decision means for deciding the timing ofthe regeneration process, when the differential pressure across thefilter exceeds a pre-determined value, and the second decision means fordeciding the timing of the regeneration process, when the PMaccumulation amount reaches or exceeds a pre-determined value. Further,a number of consecutive regenerations, namely series of busyregeneration periods whose regeneration time intervals between thedecisions for the regeneration process are shorter than a thresholdvalue, is counted, and if the number exceeds a threshold value, the PMdischarged amount from the engine is judged abnormal.

-   [Patent Document 1] Japanese Patent No. 4008866-   [Patent Document 2] Japanese Patent Laid-Open No. 2008-57443-   [Patent Document 3] Japanese Patent Laid-Open No. 2008-121631-   [Patent Document 4] Japanese Patent Laid-Open No. 2004-218558

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

When a filter regeneration process is executed, the PM accumulated atthe filter is once removed, and the PM accumulation amount increasesagain gradually after the termination of the execution of theregeneration process. In this case, when the PM accumulation amount isat a low level, a PM trapping rate of the filter (a rate of the PMamount trapped by the filter to the PM amount flown into the filter) islower compared to a state where the PM accumulation amount is at a highlevel. In other words, when the regeneration process is executed and thePM is removed, the rate of the PM that passes through the filter risesimmediately after the termination of the execution of the regenerationprocess.

In some cases a catalyst having an oxidization function, such as anoxidation catalyst and a NOx storage reduction catalyst, is carried onthe filter. In such case, if the filter temperature is raised due toexecution of the regeneration process, deactivation of the carriedcatalyst may be accelerated. With the acceleration of the deactivationof the carried catalyst, the oxidation of a reducing agent supplied tothe catalyst or HC or the like in the exhaust gas by the catalyst shouldbecome difficult.

Moreover, in case a NOx storage reduction catalyst is carried on thefilter, the amount of NOx to be stored and reduced by the NOx storagereduction catalyst is decreased during execution of the regenerationprocess due to a higher temperature of the NOx storage reductioncatalyst.

From the above reason, in case any abnormality in the filterregeneration system should occur and the execution frequency of theregeneration process become excessively high, the overall nature ofexhaust emissions may be aggravated. To prevent the aggravation of thenature of the exhaust emissions, it is required to detect correctlyabnormality in the filter regeneration system causing excessiveexecution frequency of the regeneration process.

The present invention has been conducted addressing the afore-describedproblems, with an object to provide a technology which makes it possibleto diagnose with higher accuracy whether or not abnormality occur in thefilter regeneration system causing excessive execution frequency of theregeneration process.

Means for Solving the Problems

According to the present invention, abnormality in a filter regenerationsystem that initiates execution of the regeneration process, in case anestimated PM accumulation amount at the filter reaches a pre-determinedregeneration requiring accumulation amount; or in case the pressureupstream of the filter or the differential pressure across the filterreaches a pre-determined regeneration requiring value, the value beinglarger than the pressure or the differential pressure corresponding tothe regeneration requiring accumulation amount, is diagnosed based onthe ratio of an estimated PM accumulation amount at the initiation ofthe execution of the regeneration process to the regeneration requiringaccumulation amount.

More particularly, an abnormality diagnosis system for a filterregeneration system according to the first invention is an abnormalitydiagnosis system for a filter regeneration system for executing aregeneration process of a particulate filter placed in an exhaustpassage of an internal combustion engine wherein, the filterregeneration system comprises

-   an accumulation amount estimation unit for estimating an    accumulation amount of particulate matter at the particulate filter;-   a pressure determination unit for determining a pressure upstream of    the particulate filter in the exhaust passage or a differential    pressure across the particulate filter; and-   an execution unit for the regeneration process for executing the    regeneration process to oxidize and remove the particulate matter    accumulated at the particulate filter; and-   initiates execution of the regeneration process by the execution    unit for the regeneration process, in case the accumulation amount    of the particulate matter estimated by the accumulation    amount-estimation unit reaches a pre-determined regeneration    requiring accumulation amount; or in case the pressure or the    differential pressure determined by the pressure determination unit    reaches a pre-determined regeneration requiring value, the value    being larger than the pressure or the differential pressure    corresponding to the regeneration requiring accumulation amount,    characterized in

that the abnormality diagnosis system for the filter regeneration systemcomprises:

a parameter calculation unit for calculating a ratio of: an accumulationamount of the particulate matter at the initiation of the execution ofthe regeneration process estimated by the accumulation amount estimationunit to the regeneration requiring accumulation amount as a parameterfor execution frequency of regeneration; and

a diagnosis unit for diagnosing based on the parameter for executionfrequency of regeneration whether or not abnormality is occurring in thefilter regeneration system causing excessive execution frequency of theregeneration process.

In the filter regeneration system according to the present invention,the regeneration process is usually executed when an estimated PMaccumulation amount at the filter reaches a pre-determined regenerationrequiring accumulation amount. However, when the pressure upstream ofthe filter or the differential pressure across the filter reaches apre-determined regeneration requiring value, the regeneration process isexecuted, even if the estimated PM accumulation amount has not reachedthe pre-determined regeneration requiring accumulation amount due tosome abnormality occurring in the system.

In case a regeneration process is executed as described above, when thepressure upstream of the filter or the differential pressure across thefilter reaches a pre-determined regeneration requiring value, theexecution frequency of the regeneration process becomes higher thanusual. Consequently, a ratio of an estimated value of the PMaccumulation amount at the initiation of the execution of theregeneration process (hereinafter referred to as the “PM accumulationamount at the initiation of regeneration”) to the regeneration requiringaccumulation amount is calculated according to the present invention asa parameter for execution frequency of regeneration.

At a normal operation, the estimated PM accumulation amount at theinitiation of regeneration is equal to the regeneration requiringaccumulation amount, and therefore the value of the parameter forexecution frequency of regeneration is equal to 1. On the other hand, incase a regeneration process is executed by reason that the pressureupstream of the filter or the differential pressure across the filterhas reached a pre-determined regeneration requiring value, the estimatedPM accumulation amount at the initiation of regeneration is less thanthe regeneration requiring accumulation amount, and the value of theparameter for execution frequency of regeneration varies depending on anestimated value of the PM accumulation amount at the initiation ofregeneration.

Consequently, whether or not abnormality occurs in the filterregeneration system causing excessive execution frequency of theregeneration process is diagnosed based on the thus calculated parameterfor execution frequency of regeneration.

The parameter for execution frequency of regeneration according to thepresent invention exhibits a high correlation with a change in theexecution frequency of the regeneration process due to abnormality inthe filter regeneration system. It is conceivable to calculate anexecution frequency of regeneration based on a travel distance or timeperiod from the previous termination time of the regeneration process tothe current initiation time of the regeneration process. However, thelength of such travel distance or time period varies depending onoperation conditions of the internal combustion engine during suchperiod, even when there occurs no abnormality in the filter regenerationsystem. Therefore, it is difficult to diagnose accurately abnormality inthe filter regeneration system based on the execution frequency ofregeneration calculated from the length of the travel distance or thetime period. On the contrary, the parameter for execution frequency ofregeneration according to the present invention is a value not affectedby the operation conditions of the internal combustion engine betweenthe termination time of the previous regeneration process and theinitiation time of the current regeneration process.

Consequently, whether or not abnormality occurs in the filterregeneration system causing excessive execution frequency of theregeneration process can be diagnosed with higher accuracy according tothe present invention.

According to the present invention, representing an estimated PMaccumulation amount at the initiation of regeneration by Mreq, and aregeneration requiring accumulation amount by Mnrm, a parameter forexecution frequency of regeneration can be given by Freq=Mreq/Mnrm. Inthis case, the diagnosis unit may diagnose that abnormality is occurringin the filter regeneration system causing excessive execution frequencyof the regeneration process, when the parameter for execution frequencyof regeneration Freq is smaller than a pre-determined criterion value.

In the above case, if a regeneration process is executed by reason thatthe pressure upstream of the filter or the differential pressure acrossthe filter reaches a regeneration requiring value, the parameter forexecution frequency of regeneration Freq is less than 1 (Freq<1). Inthis case the pre-determined criterion value is less than 1 and can bejudged as the upper allowable limit of the aggravation of the nature ofexhaust emissions caused by the execution of the regeneration process.

According to the second invention, an abnormality diagnosis system for afilter regeneration system for executing a regeneration process of aparticulate filter placed in an exhaust passage of an internalcombustion engine is characterized in:

that the filter regeneration system comprises an accumulation amountestimation unit for estimating an accumulation amount of particulatematter at the particulate filter;

-   a pressure determination unit for determining a pressure upstream of    the particulate filter in the exhaust passage or a differential    pressure across the particulate filter; and-   an execution unit for the regeneration process for executing the    regeneration process to oxidize and remove the particulate matter    accumulated at the particulate filter;-   initiates execution of the regeneration process by the execution    unit for the regeneration process, in case the accumulation amount    of the particulate matter estimated by the accumulation amount    estimation unit reaches a pre-determined regeneration requiring    accumulation amount; or in case the pressure or the differential    pressure determined by the pressure determination unit reaches a    pre-determined regeneration requiring value, the value being larger    than the pressure or the differential pressure corresponding to the    regeneration requiring accumulation amount; and-   terminates the execution of the regeneration process, in case, after    the initiation of the execution of the regeneration process, the    accumulation amount of the particulate matter at the particulate    filter is judged to have decreased to a pre-determined base    accumulation amount; and

that the abnormality diagnosis-system for the filter regeneration systemcomprises:

a parameter calculation unit for calculating a ratio of the differencebetween an accumulation amount of the particulate matter at theinitiation of the execution of the regeneration process estimated by theaccumulation amount estimation unit and the base accumulation amount tothe difference between the regeneration requiring accumulation amountand the base accumulation amount as a parameter for execution frequencyof regeneration; and

a diagnosis unit for diagnosing based on the parameter for executionfrequency of regeneration whether or not abnormality is occurring in thefilter regeneration system causing excessive execution frequency of theregeneration process.

At an execution of the regeneration process, if the process executionshould be continued to the lowest possible level of the PM accumulationamount at the filter, a considerably large amount of fuel is requiredfor the regeneration process. Consequently, in the filter regenerationsystem according to the present invention, the execution of theregeneration process is terminated, when the PM accumulation amount atthe filter is judged to have decreased to a pre-determined baseaccumulation amount after initiation of the execution of theregeneration process.

The term “a pre-determined base accumulation amount” above unit anamount larger than the lowest PM accumulation amount attainable by theregeneration process, and from which certain time allowance is judged tobe available until the PM accumulation amount increases again to aregeneration requiring threshold value.

Further, by the filter regeneration system according to the presentinvention, the following method may be applied to determine whether ornot the PM accumulation amount at the filter has decreased to the baseaccumulation amount. Namely, in case the execution of the regenerationprocess is initiated by reason that the PM accumulation amount at thefilter reaches a regeneration requiring accumulation amount, a PMremoval amount from the initiation of the execution of the regenerationprocess is estimated. Then, it is discriminated whether or not the valueobtained by subtracting the PM removal amount from the regenerationrequiring accumulation amount reaches the base accumulation amount.Furthermore, in case the execution of the regeneration process isinitiated by reason that the pressure upstream of the filter or thedifferential pressure across the filter reaches the regenerationrequiring value, it is discriminated whether or not the pressureupstream of the filter or the differential pressure across the filterreaches a value corresponding to the base accumulation amount.

Moreover, according to the present invention, a ratio of the differencebetween the PM accumulation amount at the initiation of the execution ofthe regeneration process and the base accumulation amount to thedifference between the regeneration requiring accumulation amount andthe base accumulation amount is calculated as a parameter for executionfrequency of regeneration. Further the diagnosis whether or notabnormality is occurring in the filter regeneration system causingexcessive execution frequency of the regeneration process is made basedon the thus calculated parameter for execution frequency ofregeneration.

The parameter for execution frequency of regeneration according to thepresent invention is similar to the first invention a value not affectedby the operation conditions of the internal combustion engine betweenthe termination time of the previous regeneration process and theinitiation time of the current regeneration process, and exhibits a highcorrelation with a change in the execution frequency of the regenerationprocess due to abnormality in the filter regeneration system.

Consequently, according to the present invention, as in the firstinvention, whether or not abnormality occurs in the filter regenerationsystem causing excessive execution frequency of the regeneration processcan be diagnosed with higher accuracy.

According to the present invention, representing an estimated PMaccumulation amount at the initiation of regeneration by Mreq, aregeneration requiring accumulation amount by Mnrm, and a baseaccumulation amount by Mbase, a parameter for execution frequency ofregeneration can be given by Freq′=(Mreq-Mbase)/(Mnrm-Mbase). In thiscase, the diagnosis unit may diagnose that abnormality is occurring inthe filter regeneration system causing excessive execution frequency ofthe regeneration process, when the parameter for execution frequency ofregeneration Freq′ is smaller than a pre-determined criterion value.

In the above case, as in the first invention, if a regeneration processis executed by reason that the pressure upstream of the filter or thedifferential pressure across the filter reaches a regeneration requiringvalue, the parameter for execution frequency of regeneration Freq isless than 1 (Freq<1). In this case the pre-determined criterion value isa value determined by a similar basis used for the criterion valueaccording to the first invention.

According to the first and second inventions, an average calculationunit may be further provided, for calculating an average value of theparameter for execution frequency of regeneration when the regenerationprocess is executed a pre-determined number of times. In this case, thediagnosis unit may diagnose whether or not abnormality is occurring inthe filter regeneration system causing excessive execution frequency ofthe regeneration process, based on the average value of the parameterfor execution frequency of regeneration calculated by the averagecalculation unit.

At an occurrence of a certain type of abnormality in the filterregeneration system, depending on the type of the abnormality, theparameter for execution frequency of regeneration may not exhibit anabnormal value at each execution of the regeneration process. Even incase such type of abnormality should occur, the abnormality can bedetected according to the above measures.

Furthermore, in the above case, the pre-determined number of times maybe so decided that the parameter for execution frequency of regenerationexhibits a value indicating abnormality at any one of the executions ofthe regeneration process repeated the pre-determined number of times,insofar as abnormality occurs in the filter regeneration systemirrespective of the type of the abnormality.

Further, the pre-determined number of times may be decided as a numberof the times of executions of the regeneration process executed from aprevious abnormality diagnosis in the filter regeneration system until atravel distance of a vehicle equipped with an internal combustion enginereaches a pre-determined travel distance. In this case, thepre-determined travel distance is so decided, that the parameter forexecution frequency of regeneration exhibits a value indicatingabnormality at any one of the executions of the regeneration process tobe carried out during the vehicle's travelling the distance in question,insofar as abnormality occurs in the filter regeneration systemirrespective of the type of the abnormality.

A method for diagnosing abnormality in a filter regeneration systemaccording to the third invention of the present invention ischaracterized in:

that the filter regeneration system executes a regeneration process of aparticulate filter placed in an exhaust passage of an internalcombustion engine; and comprises:

-   an accumulation amount estimation unit for estimating an    accumulation amount of particulate matter at the particulate filter;-   a pressure determination unit for determining a pressure upstream of    the particulate filter in the exhaust passage or a differential    pressure across the particulate filter; and-   an execution unit for the regeneration process for executing the    regeneration process to oxidize and remove the particulate matter    accumulated at the particulate filter; and-   initiates execution of the regeneration process by the execution    unit for the regeneration process, in case the accumulation amount    of the particulate matter estimated by the accumulation amount    estimation unit reaches a pre-determined regeneration requiring    accumulation amount; or in case the pressure or the differential    pressure determined by the pressure determination unit reaches a    pre-determined regeneration requiring value, the value being larger    than the pressure or the differential pressure corresponding to the    regeneration requiring accumulation amount, characterized in

that the method for diagnosing abnormality in the filter regenerationsystem comprises:

a step for calculating a ratio of an accumulation amount of theparticulate matter at the initiation of the execution of theregeneration process estimated by the accumulation amount estimationunit to the regeneration requiring accumulation amount, as a parameterfor execution frequency of regeneration; and

a step for diagnosing based on the parameter for execution frequency ofregeneration whether or not abnormality is occurring in the filterregeneration system causing excessive execution frequency of theregeneration process.

A method for diagnosing abnormality in a filter regeneration systemaccording to the fourth invention of the present invention, ischaracterized in:

that the filter regeneration system executes a regeneration process of aparticulate filter placed in an exhaust passage of an internalcombustion engine; and comprises:

-   an accumulation amount estimation unit for estimating an    accumulation amount of particulate matter at the particulate filter;-   a pressure determination unit for determining a pressure upstream of    the particulate filter in the exhaust passage or a differential    pressure across the particulate filter; and-   an execution unit for the regeneration process for executing the    regeneration process to oxidize and remove the particulate matter    accumulated at the particulate filter; and-   initiates execution of the regeneration process by the execution    unit for the regeneration process, in case the accumulation amount    of the particulate matter estimated by the accumulation amount    estimation unit reaches a pre-determined regeneration requiring    accumulation amount; or in case the pressure or the differential    pressure determined by the pressure determination unit reaches a    pre-determined regeneration requiring value, the value being larger    than the pressure or the differential pressure corresponding to the    regeneration requiring accumulation amount; and terminates the    execution of the regeneration process, in case, after the initiation    of the execution of the regeneration process, the accumulation    amount of the particulate matter at the particulate filter is judged    to have decreased to a pre-determined base accumulation amount,    characterized in

that the method for diagnosing abnormality in the filter regenerationsystem comprises:

-   initiates execution of the regeneration process by the execution    unit for the regeneration process,

a step for calculating a ratio of:

-   the difference between an accumulation amount of the particulate    matter at the initiation of the execution of the regeneration    process estimated by the accumulation amount estimation unit and the    base accumulation amount-   to the difference between the regeneration requiring accumulation    amount and the base accumulation amount, as a parameter for    execution frequency of regeneration; and

a step for diagnosing based on the parameter for execution frequency ofregeneration whether or not abnormality is occurring in the filterregeneration system causing excessive execution frequency of theregeneration process.

According to the third and fourth invention inventions, as in the firstand second inventions, whether or not abnormality occurs in the filterregeneration system causing excessive execution frequency of theregeneration process can be diagnosed with higher accuracy.

Effect of the Invention

According to the present invention, whether or not abnormality occurs inthe filter regeneration system causing excessive execution frequency ofthe regeneration process can be diagnosed with higher accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an outline constitution of aninternal combustion engine and an intake-exhaust system thereof pursuantto Embodiment 1;

FIG. 2 is graphs showing the time courses of the estimated PMaccumulation amount and the upstream/downstream differential pressurepursuant to Embodiment 1;

FIG. 3 is a flowchart of the regeneration process flow pursuant toEmbodiment 1;

FIG. 4 is a graph showing the relationship between the PM trapping rateof the filter and the PM accumulation amount at the filter;

FIG. 5 is the first graph showing the relationship between a parameterfor execution frequency of regeneration and the degree of aggravation ofthe nature of exhaust emissions pursuant to Embodiment 1;

FIG. 6 is the second graph showing the relationship between a parameterfor execution frequency of regeneration and the degree of aggravation ofthe nature of exhaust emissions pursuant to Embodiment 1;

FIG. 7 is a graph showing the relationship between a parameter forexecution frequency of regeneration and the exhaust gas aggravationfactor pursuant to Embodiment 1;

FIG. 8 is a flowchart of the abnormality diagnosis flow in the filterregeneration system pursuant to Embodiment 1;

FIG. 9 is graphs showing the time courses of the estimated PMaccumulation amount and the upstream/downstream differential pressurepursuant to Embodiment 2;

FIG. 10 is a flowchart of the regeneration process flow pursuant toEmbodiment 2;

FIG. 11 is a flowchart of the abnormality diagnosis flow in the filterregeneration system pursuant to Embodiment 2; and

FIG. 12 is a flowchart of the abnormality diagnosis flow in the filterregeneration system pursuant to Embodiment 3.

DESCRIPTION OF SYMBOLS

-   -   1. internal combustion engine    -   2. cylinder    -   4. intake passage    -   6. exhaust passage    -   9. oxidation catalyst    -   10. particulate filter    -   13. fuel addition valve    -   14. NOx storage reduction catalyst    -   15. upstream temperature sensor    -   16. downstream temperature sensor    -   17. differential pressure sensor    -   20. ECU    -   21. crank position sensor    -   22. accelerator sensor

BEST MODE FOR CARRYING OUT THE INVENTION

Specific embodiments of the abnormality diagnosis system and method fordiagnosing abnormality in the filter regeneration system according tothe present invention will now be described with reference to thefigures. The sizes, materials, shapes, relative arrangements and thelike of the components described in the following embodiments are notintended to limit the technical scope of the invention solely thereto,unless otherwise specifically set forth herein.

Embodiment 1 Outline Constitution of Internal Combustion Engine andIntake-Exhaust System Thereof

FIG. 1 is a schematic representation of an outline constitution of aninternal combustion engine and an intake-exhaust system thereof pursuantto the present embodiment. The internal combustion engine 1 is a dieselengine for driving a vehicle and has 4 cylinders 2. Each cylinder 2 isprovided with a fuel injection valve 3 for injecting a fuel directlyinto the cylinder 2.

The internal combustion engine 1 is connected with an intake manifold 5and an exhaust manifold 7. The intake manifold 5 is connected with anend of the intake passage 4. The exhaust manifold 7 is connected with anend of the exhaust passage 6.

The intake passage 4 is provided with a compressor housing 8 a of aturbocharger 8. The exhaust passage 6 is provided with a turbine housing8 b of the turbocharger 8. While, the intake manifold 5 and the exhaustmanifold 7 are interconnected by an EGR passage 23. The EGR passage 23is provided with an EGR valve 24 to regulate the flow rate of the EGRgas.

An air flowmeter 11 is provided in the intake passage 4 upstream of thecompressor housing 8 a. A throttle valve 12 is provided in the intakepassage 4 downstream of the compressor housing 8 a.

An oxidation catalyst 9 is provided in the exhaust passage 6 downstreamof the turbine housing 8 b. A filter 10 for trapping the PM in theexhaust gas is provided in the exhaust passage 6 downstream of theoxidation catalyst 9. The filter 10 carries a NOx storage reductioncatalyst 14. A fuel addition valve 13 for adding a fuel into the exhaustgas as a reducing agent is provided in the exhaust passage 6 downstreamof the turbine housing 8 b and upstream of the oxidation catalyst 9.

An oxygen concentration sensor 18 and an upstream temperature sensor 15are provided in the exhaust passage 6 downstream of the oxidationcatalyst 9 and upstream of the filter 10. A downstream temperaturesensor 16 is provided in the exhaust passage 6 downstream of the filter10. Further, a differential pressure sensor 17 for measuring thedifferential pressure across the filter 10 is provided in the exhaustpassage 6.

An electronic control unit (ECU) 20 is annexed to the internalcombustion engine 1. The ECU 20 is a unit for regulating operationconditions of the internal combustion engine 1. The ECU 20 iselectrically connected with the air flowmeter 11, the oxygenconcentration sensor 18, the upstream temperature sensor 15, thedownstream temperature sensor 16, the differential pressure sensor 17, acrank position sensor 21, and an accelerator position sensor 22. Thecrank position sensor 21 detects the crank angle of the internalcombustion engine 1. The accelerator position sensor 22 detects theaccelerator pedal angle of a vehicle equipped with the internalcombustion engine 1. Output signals of the respective sensors are inputto the ECU 20.

The ECU 20 derives the temperature of the filter 10 from the outputvalues of the upstream temperature sensor 15 and the downstreamtemperature sensor 16. The ECU 20 derives the engine rotation speed fromthe output values of the crank position sensor 21. The ECU 20 derivesthe engine load on the internal combustion engine 1 from the outputvalue of the accelerator position sensor 22.

The respective fuel injection valves 3, the throttle valve 12, the EGRvalve 24 and the fuel addition valve 13 are electrically connected withthe ECU 20 and regulated by the ECU 20.

(Regeneration Process)

According to the present embodiment, a regeneration process is executedto remove the PM accumulated at the filter 10. The regeneration processis carried out by adding the fuel through the fuel addition valve 13.The fuel added through the fuel addition valve 13 is fed to theoxidation catalyst 9 and the NOx storage reduction catalyst 14, andoxidized by the catalysts. In such case, the temperature of the filter10 is raised by the generated oxidation heat, so that the PM is oxidizedand removed.

In the regeneration process, the fuel addition amount through the fueladdition valve 13 is so regulated that the temperature of the filter 10is controlled to a target temperature to be decided as a temperature, atwhich oxidation of the PM is possible and erosion or breakage of thefilter 10 can be suppressed. Moreover, instead of the fuel additionthrough the fuel addition valve 13, an auxiliary fuel injection throughthe fuel injection valves 3 of the internal combustion engine 1 may becarried out at a pre-determined timing to feed the fuel to the oxidationcatalyst 9 and the NOx storage reduction catalyst 14.

Now, the timings of initiation and termination of the execution of theregeneration process pursuant to the present embodiment will bedescribed with reference to FIG. 2. FIG. 2 is graphs showing the timecourses of the estimated PM accumulation amount at the filter 10(hereinafter referred to simply as the “estimated PM accumulationamount”) Me and the differential pressure across the filter 10(hereinafter referred to simply as the “differential pressure”) ΔPpursuant to the present embodiment. The estimated PM accumulation amountMe is calculated by the ECU 20 based on the history of the operationconditions of the internal combustion engine 1, the temperature of thefilter 10, and the like, since the termination of the previousregeneration process. The differential pressure ΔP is a measured valueof the differential pressure sensor 17.

The differential pressure ΔP increases in accordance with the increaseof the PM accumulation amount at the filter 10. Pursuant to the presentembodiment, the execution of the regeneration process is initiated, whenthe estimated PM accumulation amount Me reaches a pre-determinedregeneration requiring accumulation amount Mnrm, or the differentialpressure ΔP reaches a pre-determined regeneration requiring differentialpressure ΔPreq.

The pre-determined regeneration requiring accumulation amount Mnrm andregeneration requiring differential pressure ΔPreq are values smallerthan such PM accumulation amount or differential pressure, as increasesthe back pressure beyond the allowable range of the effect on theoperation of the internal combustion engine 1, etc. The regenerationrequiring differential pressure ΔPreq is a value larger than thedifferential pressure corresponding to the regeneration requiringaccumulation amount Mnrm.

In other words, the estimated PM accumulation amount Me reaches theregeneration requiring accumulation amount Mnrm, before the differentialpressure ΔP reaches the regeneration requiring differential pressureΔPreq. Consequently, normally as the timing represented by t1 in FIG. 2,the execution of the regeneration process is initiated, when theestimated PM accumulation amount Me reaches the regeneration requiringaccumulation amount Mnrm.

However, in case some abnormality should occur in the filterregeneration system, the differential pressure ΔP reaches occasionallythe regeneration requiring differential pressure ΔPreq, before theestimated PM accumulation amount Me reaches the regeneration requiringaccumulation amount Mnrm. In such a case, as the timing represented byt2 in FIG. 2, the execution of the regeneration process is initiated,when the differential pressure ΔP reaches the regeneration requiringdifferential pressure ΔPreq.

While, the initiated execution of the regeneration process is terminatedpursuant to the present embodiment, when a regeneration processexecution time Δ_(tre) calculated by the ECU20 has expired. Theregeneration process execution time Δ_(tre) is a time duration, in whichthe PM accumulation amount at the filter 10 is judged to be reduced to apossible minimal amount.

In case the execution of the regeneration process is initiated by reasonthat the estimated PM accumulation amount Me has reached theregeneration requiring accumulation amount Mnrm, the regenerationprocess execution time Δ_(tre) is calculated based on the regenerationrequiring accumulation amount Mnrm and the flow rate, oxygenconcentration, etc. of the exhaust gas flown into the filter 10; and incase the execution of the regeneration process is initiated by reasonthat the differential pressure ΔP has reached the regeneration requiringdifferential pressure ΔPreq, the same is calculated based on theregeneration requiring differential pressure ΔPreq, and the flow rate,oxygen concentration, etc. of the exhaust gas flown into the filter 10.

(Regeneration Process Flow)

The regeneration process flow pursuant to the present embodiment will bedescribed with reference to the flowchart shown in FIG. 3. The flow isstored in advance in the ECU 20 and executed by the ECU 20.

According to the flow, initially at the step S101, the estimated PMaccumulation amount Me is calculated.

Next, at the step S102, whether or not the estimated PM accumulationamount Me has reached the regeneration requiring accumulation amountMnrm is discriminated. If affirmatively judged at the step S102, thenthe task of the step S103 is executed; and if negatively judged, thenthe task of the step S107 is executed.

At the step S107, whether or not the differential pressure ΔP hasreached the regeneration requiring differential pressure ΔPreq isdiscriminated. At this step S107, negative judgment is normal, and inthis case the execution of the flow is terminated. However, if someabnormality should occur in the filter regeneration process system, thestep S107 occasionally judges affirmatively.

At the step S103, the regeneration process execution time Δ_(tre) iscalculated for the execution that is initiated at the next step S104.

Next, at the step S104, the execution of the regeneration process isinitiated.

Next, at the step S105, whether or not the regeneration processexecution time Δ_(tre) has expired since the initiation of the executionof the regeneration process is discriminated. If affirmatively judged atthe step S105, the execution of the regeneration process is terminatedat the step S106.

Meanwhile, the ECU 20, which executes the task of the step S101 of theregeneration process flow, is equivalent to the accumulation amountestimation unit according to the present invention. Further, thedifferential pressure sensor 17 is equivalent to the pressuredetermination unit according to the present invention, and the ECU 20,which executes the regeneration process by adding the fuel through thefuel addition valve 13, is equivalent to the execution unit for theregeneration process according to the present invention.

Further, pursuant to the present embodiment, instead of the differentialpressure across the filter 10, the pressure in the exhaust passage 6upstream of the filter 10 may be used as a criterion value forinitiation of the execution of the regeneration process. In this case,when the pressure in the exhaust passage 6 upstream of the filter 10reaches a regeneration requiring pressure, the execution of theregeneration process is initiated. The regeneration requiring pressureis higher than the differential pressure corresponding to theregeneration requiring accumulation amount Mnrm.

(Relationship Between Execution Frequency of Regeneration Process andExhaust Emissions)

In case, as described above, some abnormality should occur in the filterregeneration system, and the execution of the regeneration process isinitiated by reason that the differential pressure ΔP reaches theregeneration requiring differential pressure ΔPreq, the execution of theregeneration process is initiated earlier than in a normal state.Consequently, the execution frequency of the regeneration processbecomes higher. In this case compared to a normal state, where theexecution of the regeneration process will be initiated by reason thatthe estimated PM accumulation amount Me reaches the regenerationrequiring accumulation amount Mnrm, the execution of the regenerationprocess is initiated at a stage that the actual PM accumulation amountis still at lower level.

FIG. 4 is a graph showing the relationship between the PM trapping rateof the filter 10 and the PM accumulation amount at the filter 10. InFIG. 4, the vertical axis represents the PM trapping rate of the filter10 and the horizontal axis represents the PM accumulation amount at thefilter 10. As shown in FIG. 4, the PM trapping rate of the filter 10 islower at a low PM accumulation amount than at a high PM accumulationamount.

If the execution frequency of the regeneration process becomes higher,the frequency that the PM accumulation amount at the filter 10 stays inthe range where the PM trapping rate of the filter 10 is relatively low,becomes also higher. Consequently, the rate of the PM that passesthrough the filter 10 increases.

Further, since the temperature of the filter 10 rises during theexecution of the regeneration process, the degradation of the NOxstorage reduction catalyst 14 carried on the filter 10 is accelerated byfrequent execution of the regeneration process. As the result, the NOxstorage reduction ability or the oxidation ability of the NOx storagereduction catalyst 14 is compromised.

Furthermore, since the temperature of the NOx storage reduction catalyst14 also rises during the execution of the regeneration process, thestorage of NOx in the NOx storage reduction catalyst 14 becomesdifficult during such execution. As the result, more NOx is emittedwithout being stored and reduced by the NOx storage reduction catalyst14.

As described above, in case some abnormality should occur in the filterregeneration system leading to excessive execution frequency of theregeneration process, the overall nature of exhaust emissions may beaggravated. Consequently, pursuant to the present embodiment, whether ornot abnormality occurs in the filter regeneration system causingexcessive execution frequency of the regeneration process is diagnosedbased on a parameter for execution frequency of regeneration to bedescribed below.

(Diagnosis of Abnormality in Filter Regeneration System)

The estimated PM accumulation amount at the initiation of theregeneration is represented by Mreq. As shown in FIG. 2, in a normalstate, where the execution of the regeneration process is initiated byreason that the estimated PM accumulation amount Me reaches theregeneration requiring accumulation amount Mnrm, the relation Mreq=Mnrmholds logically. While, in case the execution of the regenerationprocess is initiated by reason that the differential pressure ΔP reachesthe regeneration requiring differential pressure ΔPreq, the relationMreq<Mnrm holds.

So, pursuant to the present embodiment, a parameter for executionfrequency of regeneration is calculated as Freq=Mreq/Mnrm. The value ofthe parameter for execution frequency of regeneration Freq is 1 in anormal state. While, in case the execution of the regeneration processis initiated by reason that the differential pressure ΔP reaches theregeneration requiring differential pressure ΔPreq, namely, in case theexecution frequency of the regeneration process is higher than in anormal state, the value of the parameter for execution frequency ofregeneration Freq is less than 1.

The relationship between the parameter for execution frequency ofregeneration Freq and the degree of aggravation of the nature of exhaustemissions will be described with reference to FIG. 5 and FIG. 6. In FIG.5, the vertical axis represents the degree of aggravation of the natureof exhaust emissions in connection with the PM trapping rate of thefilter 10. While, in FIG. 6, the vertical axis represents the degree ofaggravation of the nature of exhaust emissions in connection with theexecution of the regeneration process. The horizontal axes in FIG. 5 andFIG. 6 represent the parameter for execution frequency of regenerationFreq.

In case the parameter for execution frequency of regeneration Freqbecomes small, the frequency that the PM accumulation amount stays inthe range where the PM trapping rate is relatively low becomes higher,and therefore the degree of aggravation of the nature of exhaustemissions becomes higher in connection therewith as shown in FIG. 5.While, as shown in FIG. 6, in case the parameter for execution frequencyof regeneration Freq becomes small, due to acceleration of degradationof the NOx storage reduction catalyst 14 and increase in occasions ofpoorer storage of NOx in the NOx storage reduction catalyst 14, thedegree of aggravation of the nature of exhaust emissions in connectionwith the execution of the regeneration process becomes higher.

Now, a value taking into consideration features of both the degree ofaggravation of the nature of exhaust emissions shown in FIG. 5 and FIG.6 is used as an exhaust gas aggravation factor. The relationship betweenthe exhaust gas aggravation factor and the parameter for executionfrequency of regeneration Freq becomes as shown in FIG. 7. In FIG. 7 thevertical axis represents the exhaust gas aggravation factor, and thehorizontal axis represents the parameter for execution frequency ofregeneration Freq. The relationship can be found through an experiment,etc.

A value of the parameter for execution frequency of regeneration Freq,at which the exhaust gas aggravation factor coincides with the upperlimit of an allowable range thereof, is determined as a pre-determinedcriterion value Freq0. As the result, in case a parameter for executionfrequency of regeneration Freq is below the pre-determined criterionvalue Freq0, it can be so judged that abnormality is occurring in thefilter regeneration system causing execution frequency of theregeneration process so high as to aggravate the nature of exhaustemissions beyond the limit of the allowable range.

(Flow for Diagnosis of Abnormality in Filter Regeneration System)

The flow for diagnosis of abnormality in the filter regeneration systempursuant to the present embodiment will be described with reference tothe flow chart shown in FIG. 8. The flow is stored in advance in the ECU20 and executed by the ECU 20.

According to the flow, initially at the step S201, it is discriminatedwhether or not an initiation request for the execution of theregeneration process has been issued. The initiation request for theexecution of the regeneration process is issued when the estimated PMaccumulation amount Me reaches the regeneration requiring accumulationamount Mnrm, or the differential pressure ΔP reaches the regenerationrequiring differential pressure ΔPreq. If affirmatively judged at thestep S201, then the task of the step S202 is executed.

At the step S202, the estimated PM accumulation amount at the initiationof the regeneration Mreq is read in.

Next, at the step S203, the parameter for execution frequency ofregeneration Freq is calculated from the estimated PM accumulationamount at the initiation of the regeneration Mreq read in at the stepS202 and the regeneration requiring accumulation amount Mnrm.

Next, at the step S204, it is discriminated whether or not the parameterfor execution frequency of regeneration Freq is smaller than apre-determined criterion value Freq0.

If affirmatively judged at the step S204, next at the step S205 it is sodiagnosed that abnormality is occurring in the filter regenerationsystem causing excessive execution frequency of the regenerationprocess. If negatively judged at the step S204, next at the step S206 itis so diagnosed that the filter regeneration system is in a normalstate.

The parameter for execution frequency of regeneration Freq pursuant tothe present embodiment is a value that varies due to the shift of theinitiation timing of the execution of the regeneration process from thenormal timing, in case abnormality occurs in the filter regenerationsystem. However, the parameter for execution frequency of regenerationFreq is not a value that varies in accordance with operation conditionsof the internal combustion engine between the previous termination timeof the regeneration process and the current initiation time of theregeneration process.

Consequently, as described above, by diagnosing abnormality based on theparameter for execution frequency of regeneration Freq pursuant to thepresent embodiment, whether or not abnormality occurs in the filterregeneration system causing excessive execution frequency of theregeneration process can be diagnosed with higher accuracy.

Meanwhile, the ECU 20, which executes the task of the step S203 of theabnormality diagnosis flow, is equivalent to the parameter calculationunit according to the present invention. Further, the ECU 20, whichexecutes the tasks of the steps S203 through S206 of the abnormalitydiagnosis flow is equivalent to the diagnosis unit according to thepresent invention.

Embodiment 2

An outline constitution of an internal combustion engine and anintake-exhaust system thereof pursuant to the present embodiment issimilar to Embodiment 1.

(Regeneration Process)

Pursuant to the present embodiment, as in Embodiment 1, the regenerationprocess is carried out by adding the fuel through the fuel additionvalve 13. Now, the timings of initiation and termination of theexecution of the regeneration process pursuant to the present embodimentwill be described with reference to FIG. 9. FIG. 9 is graphs showing thetime courses of the estimated PM accumulation amount Me and thedifferential pressure ΔP pursuant to the present embodiment.

The initiation timing of the execution of the regeneration processpursuant to the present embodiment is similar to Embodiment 1. Namely,the execution of the regeneration process is initiated, when theestimated PM accumulation amount Me reaches a pre-determinedregeneration requiring accumulation amount Mnrm (t1 in FIG. 9), or thedifferential pressure ΔP reaches a pre-determined regeneration requiringdifferential pressure ΔPreq (t2 in FIG. 9).

However, the termination timing of the execution of the regenerationprocess is different from Embodiment 1. Pursuant to the presentembodiment, in order to suppress deterioration of the fuel economy bythe execution of the regeneration process, the execution of theregeneration process is terminated, when the PM accumulation amount atthe filter 10 is judged to have decreased to a pre-determined baseaccumulation amount Mbase after the initiation of the execution of theregeneration process.

More particularly, in case the execution of the regeneration process isinitiated by reason that an estimated PM accumulation amount Me reachesa regeneration requiring accumulation amount Mnrm, a decrease in the PMaccumulation amount since the initiation of the execution of theregeneration process is estimated. Then, when the value obtained bysubtracting the decrease from the regeneration requiring accumulationamount Mnrm reaches the base accumulation amount Mbase, the execution ofthe regeneration process is terminated. Meanwhile, in case the executionof the regeneration process is initiated by reason that the differentialpressure ΔP reaches the regeneration requiring differential pressureΔPreq, the execution of the regeneration process is terminated, when thedifferential pressure ΔP reaches the base differential pressure ΔPbasethat is a differential pressure corresponding to the base accumulationamount Mbase.

The pre-determined base accumulation amount Mbase above means an amountlarger than the lowest PM accumulation amount attainable by theregeneration process, and from which certain time allowance is judged tobe available until the PM accumulation amount increases again to aregeneration requiring threshold value. The pre-determined baseaccumulation amount Mbase is determined in advance according to anexperiment, etc.

(Regeneration Process Flow)

The regeneration process flow pursuant to the present embodiment will bedescribed with reference to the flow chart shown in FIG. 10. The flow isstored in advance in the ECU 20 and executed by the ECU 20. Since thetasks of the steps S101, S102, S106 and S107 of this flow are same as inthe flow in FIG. 3, the descriptions will not be reiterated.

Pursuant to the present flow, if affirmatively judged at the step S102,next the task of S303 is executed. At the step S303, the execution ofthe regeneration process is initiated.

Then at the step S304, a decrease ΔMd in the PM accumulation amountafter the initiation of the execution of the regeneration process iscalculated. The decrease ΔMd is calculated based on the flow rate,oxygen concentration, etc. of the exhaust gas flown into the filter 10,the temperature of the filter 10, and the like during the execution ofthe regeneration process.

Next, at the step S305, a currently applicable estimated PM accumulationamount Me can be calculated by subtracting the decrease ΔMd in the PMaccumulation amount calculated at the step S304 from the regenerationrequiring accumulation amount Mnrm.

Next, at the step S306, whether or not the estimated PM accumulationamount Me calculated at the step S305 has decreased to or below the baseaccumulation amount Mbase is discriminated. If affirmatively judged atthe step S306, then the task of the step S106 is executed. While, ifnegatively judged at the step S306, the task of the step S304 isrepeated.

Further, according to the present flow, if affirmatively judged at thestep S107, then the task of the step S308 is executed. At the step S308,the execution of the regeneration process is initiated.

Next, at the step S309, whether or not the current differential pressureΔP has decreased to or below the base differential pressure ΔPbase isdiscriminated. If affirmatively judged at the step S309, then the taskof the step S106 is executed. While, if negatively judged at the stepS309, the task of the step S308 is repeated.

Meanwhile, pursuant to the present embodiment, instead of thedifferential pressure across the filter 10, the pressure in the exhaustpassage 6 upstream of the filter 10 may be used as a criterion value forinitiation and termination of the execution of the regeneration process.In this case, when the pressure in the exhaust passage 6 upstream of thefilter 10 reaches a regeneration requiring pressure, the execution ofthe regeneration process is initiated, wherein the regenerationrequiring pressure is higher than the differential pressurecorresponding to the regeneration requiring accumulation amount Mnrm.Further, the execution of the regeneration process is terminated, whenthe pressure upstream of the filter 10 reaches the base pressure,wherein the base pressure is the pressure corresponding to the baseaccumulation amount Mbase.

(Diagnosis of Abnormality in Filter Regeneration System)

Pursuant to the present embodiment, whether or not abnormality occurs inthe filter regeneration system causing excessive execution frequency ofthe regeneration process is diagnosed also based on a parameter forexecution frequency of regeneration, wherein the parameter for executionfrequency of regeneration is calculated asFreq′=(Mreq−Mbase)/(Mnrm−Mbase).

The value of the parameter for execution frequency of regeneration Freq′pursuant to the present embodiment is also 1 in a normal state. While,in case the execution of the regeneration process is initiated by reasonthat the differential pressure ΔP reaches the regeneration requiringdifferential pressure ΔPreq, namely, in case the execution frequency ofthe regeneration process is higher than in a normal state, the value isless than 1.

While, similar to the parameter for execution frequency of regenerationFreq pursuant to Embodiment 1, the parameter for execution frequency ofregeneration Freq′ is also correlative to the exhaust gas aggravationfactor, and the relationship can be found through an experiment, etc.

A value of the parameter for execution frequency of regeneration Freq′,at which the exhaust gas aggravation factor coincides with the upperlimit of an allowable range thereof, is determined as a pre-determinedcriterion value Freq0′. As the result, in case a parameter for executionfrequency of regeneration Freq′ is below the pre-determined criterionvalue Freq0′, it can be so judged that abnormality is occurring in thefilter regeneration system causing execution frequency of theregeneration process so high as to aggravate the nature of exhaustemissions beyond the limit of the allowable range.

(Flow for Diagnosis of Abnormality in Filter Regeneration System)

The flow for diagnosis of abnormality in the filter regeneration systempursuant to the present embodiment will be described with reference tothe flow chart shown in FIG. 11. The flow is stored in advance in theECU 20 and executed by the ECU 20. Meanwhile, the present flow isidentical with the flow in FIG. 8, except that steps S203 and S204therein are replaced by the steps S403 and S404. Consequently, only thetasks of the steps S403 and S404 will be described.

Pursuant to the present flow, at the step S403, the parameter forexecution frequency of regeneration Freq′ is calculated from theestimated PM accumulation amount at the initiation of the regenerationMreq read in at the step S202, the regeneration requiring accumulationamount Mnrm and the base accumulation amount Mbase.

Next, at the step S404, it is discriminated whether or not the parameterfor execution frequency of regeneration Freq′ is smaller than apre-determined criterion value Freq0′.

If affirmatively judged at the step S404, next at the step S205 it is sodiagnosed that abnormality is occurring in the filter regenerationsystem causing excessive execution frequency of the regenerationprocess. If negatively judged at the step S404, next at the step S206 itis so diagnosed that the filter regeneration system is in a normalstate.

The parameter for execution frequency of regeneration Freq′ pursuant tothe present embodiment is also a value that varies due to the shift ofthe initiation timing of the execution of the regeneration process fromthe normal timing, in case abnormality occurs in the filter regenerationsystem. However, similar to the parameter for execution frequency ofregeneration Freq pursuant to Embodiment 1, the parameter for executionfrequency of regeneration Freq′ is not a value that varies in accordancewith operation conditions of the internal combustion engine between theprevious termination time of the regeneration process and the currentinitiation time of the regeneration process.

Consequently, as described above, by diagnosing abnormality based on theparameter for execution frequency of regeneration Freq′ pursuant to thepresent embodiment, whether or not abnormality occurs in the filterregeneration system causing excessive execution frequency of theregeneration process can be diagnosed with higher accuracy.

Meanwhile, the ECU 20, which executes the task of the step S403 of theabnormality diagnosis flow, is equivalent to the parameter calculationunit according to the present invention. Further, the ECU 20, whichexecutes the tasks of the steps S404, S205 and S206 of the abnormalitydiagnosis flow, is equivalent to the diagnosis unit according to thepresent invention.

Embodiment 3

The outline constitution of an internal combustion engine and anintake-exhaust system thereof pursuant to the present embodiment issimilar to Embodiment 1. Further, the regeneration process pursuant tothe present embodiment is executed according to a method similar toEmbodiment 1.

(Diagnosis of Abnormality in Filter Regeneration System)

Examples of the abnormality in the filter regeneration system causingexcessive execution frequency of the regeneration process include thefollowing (a) to (e):

(a) an excessive PM discharge amount from an internal combustion engine1;

(b) decrease in the continuous regeneration ability of a NOx storagereduction catalyst 14;

(c) disorder in the regeneration process;

(d) increase of the accumulation amount of materials poorly removable bythe regeneration process (e.g. ashes); and

(e) failure of a differential pressure sensor 17 (faulty offset orfaulty sensitivity).

The (a) above is caused by poor sprayability of the fuel injection valve3, excessive amount of the EGR gas, etc. The (b) above is caused bydecrease in the oxidation function of the NOx storage reduction catalyst14 through degradation. The continuous regeneration referred to abovemeans that the PM is oxidized and removed, when the regeneration processis not executed, by the temperature increase of the exhaust gas. The (c)above is caused by a failure of a fuel addition valve 13 or atemperature sensor 15, 16. If any of the abnormality (a) to (c) shouldoccur, the conditions of the filter 10 become different from thoseexpected during the execution of the regeneration process, and theresidual PM increases.

Pursuant to the present embodiment, as in Embodiment 1, a parameter forexecution frequency of regeneration is calculated as Freq=Mreq/Mnrm. Theparameter for execution frequency of regeneration Freq is calculated ateach execution of the regeneration process.

If the abnormality (d) or (e) should occur, the output signal of thedifferential pressure sensor 17 continues to indicate an abnormal value.Consequently, the regeneration process may be initiated each time byreason that the differential pressure ΔP reaches the regenerationrequiring differential pressure ΔPreq. As the result, at each executionof the regeneration process the calculated parameter for executionfrequency of regeneration Freq indicates an abnormal value (a value lessthan a criterion value Freq0). In this case the diagnosis of abnormalityin the filter regeneration system can be carried out based on theparameter for execution frequency of regeneration Freq calculated at asingle execution of the regeneration process.

While, the parameter for execution frequency of regeneration Freqindicates an abnormal value due to occurrence of any of the abnormality(a) to (c), because increase of the PM accumulation amount at the filter10 is faster than in a normal state, and the PM accumulation amountbecomes excessive. In other words, with the excessive PM accumulationamount, the differential pressure ΔP reaches the regeneration requiringdifferential pressure ΔPreq, and the regeneration process is initiated.

In this case, since the PM accumulation amount is significantlydecreased by the execution of the regeneration process, even if at asingle execution of the regeneration process the parameter for executionfrequency of regeneration Freq should indicate an abnormal value, theparameter for execution frequency of regeneration Freq may notnecessarily indicate an abnormal value at the following executions ofthe regeneration process. Therefore, when a plurality of executions ofthe regeneration process are carried out, at some executions theparameter for execution frequency of regeneration Freq indicatesabnormal, but at other executions normal. Consequently, in case ofabnormality due to (a) to (c) above, it is difficult to diagnoseaccurately the abnormality in the filter regeneration system solelybased on the parameter for execution frequency of regeneration Freqcalculated at a single execution of the regeneration process.

It is possible in rare cases that the parameter for execution frequencyof regeneration Freq indicates an abnormal value due to a noise at adifferential pressure sensor or the like, even if there occurs actuallyno abnormality in the filter regeneration system. An occurrence rate ofsuch abnormal value of the parameter for execution frequency ofregeneration Freq is quite low compared to the occurrence rate of theabnormal value of the parameter for execution frequency of regenerationFreq due to the abnormality according to (a) to (c) above. However, ifabnormality in the filter regeneration system is diagnosed solely basedon the parameter for execution frequency of regeneration Freq calculatedat a single execution of the regeneration process, such exceptional casemay be diagnosed erroneously as abnormality in the system.

Consequently, pursuant to the present embodiment, the parameter forexecution frequency of regeneration Freq is calculated at each executionof the regeneration process, and after completion of a pre-determinednumber of the executions of the regeneration process an average value ofthe parameter for execution frequency of regeneration is calculated.Then, based on the average value, whether or not abnormality isoccurring in the filter regeneration system causing excessive executionfrequency of the regeneration process is diagnosed.

The pre-determined number means herein such a number of executions ofthe regeneration process, that, in case abnormality in the filterregeneration system should occur, irrespective of the type of theabnormality, at any one of the pre-determined number of executions ofthe regeneration process the parameter for execution frequency ofregeneration indicates an abnormal value. The pre-determined number canbe determined in advance by an experiment.

By the above measures, abnormality can be detected irrespective of thetype of the abnormality in the filter regeneration system. Consequently,whether or not abnormality is occurring in the filter regenerationsystem can be diagnosed with higher accuracy.

(Flow for Diagnosis of Abnormality in Filter Regeneration System)

The flow for diagnosis of abnormality in the filter regeneration systempursuant to the present embodiment will be described with reference tothe flow chart shown in FIG. 12. The flow is stored in advance in theECU 20 and executed by the ECU 20. Meanwhile, the present flow isidentical with the flow in FIG. 8, except that step S204 therein isreplaced by the steps S504 to S507. Consequently, only the tasks for thesteps S504 to S507 will be described.

Pursuant to the present flow, at the step S504 the parameter forexecution frequency of regeneration Freq calculated at the step S203 isstored.

Next, at the step S505, it is discriminated whether or not the number ofexecutions of the regeneration process n after the previous execution ofthe abnormality diagnosis for the filter regeneration system reached apre-determined number n0. If affirmatively judged at the step S505, thenthe task of the step S506 is executed, and if negatively judged theexecution of the flow is stopped awhile.

At the step S506, the average value Freq_av of the values of theparameter for execution frequency of regeneration after n-timeexecutions of the regeneration process is calculated.

Next, at the step S507, it is discriminated whether or not the averagevalue Freq_av of the parameter for execution frequency of regenerationis smaller than a pre-determined criterion value Freq_av0. In this case,similar to the pre-determined criterion value Freq0 pursuant toEmbodiment 1, an average value of the parameter for execution frequencyof regeneration Freq_av, at which the exhaust gas aggravation factorcoincides with the upper limit of an allowable range of the exhaust gasaggravation factor, is determined as a pre-determined criterion valueFreq_av0.

If affirmatively judged at the step S507, next at the step S205 it is sodiagnosed that abnormality is occurring in the filter regenerationsystem causing excessive execution frequency of the regenerationprocess. If negatively judged at the step S507, next at the step S206 itis so diagnosed that the filter regeneration system is in a normalstate.

Meanwhile, pursuant to the present embodiment, the ECU 20, whichexecutes the tasks of the step S506 of the abnormality diagnosis flow,is equivalent to the average calculation unit according to the presentinvention.

(Variations)

Pursuant to the above-described method for diagnosing abnormality, thepre-determined number n0 as the number of executions of the regenerationprocess required for calculating an average value Freq_av of theparameter for execution frequency of regeneration is determined inadvance based on an experiment. However, the pre-determined number n0may be determined from the historical relationship between the traveldistance of the vehicle and the timing at which the parameter forexecution frequency of regeneration Freq became abnormal.

In this case, when an abnormal value of the parameter for executionfrequency of regeneration probably caused by the abnormality (a) to (c)appears, a vehicle travel distance, within which the abnormal value canbe detected, is determined and used as a pre-determined travel distance.Then based on the distance the vehicle has travelled between theprevious execution of the regeneration process and the current executionof the regeneration process, and the pre-determined travel distance, thenumber of the executions of the regeneration process to be executedduring traveling the pre-determined travel distance is estimated. Theestimated value is used as the pre-determined number n0.

In this case, the pre-determined travel distance is a travel distance,within which at least one abnormal value of the parameter for executionfrequency of regeneration should appear, and may be a travel distance,within which a plurality of abnormal values of the parameter forexecution frequency of regeneration appear. Namely, the pre-determinedtravel distance is a travel distance, which gives the value of thepre-determined number n0 allowing the diagnosis of the abnormality inthe filter regeneration system with adequate accuracy.

Furthermore, the method for diagnosing abnormality in the filterregeneration system pursuant to the present embodiment may be applied tothe case, in which the regeneration process is executed according to amethod similar to Embodiment 2. In such case, the abnormality diagnosisis made based on the average value Freq′_av of the values of theparameter for execution frequency of regenerationFreq′(Mreq−Mbase)/(Mnrm−Mbase)) with respect to the pre-determinednumber of executions of the regeneration process, in replace of theaverage value Freq_av of the values of the parameter for executionfrequency of regeneration Freq(=Mreq/Mnrm) with respect to thepre-determined number of executions of the regeneration process.

In case it is so diagnosed by the method for diagnosing abnormalitypursuant to any of the embodiments, that abnormality in the filterregeneration system is occurring, a driver of the vehicle equipped withthe internal combustion engine 1 may be notified to that effect.

1. An abnormality diagnosis system for a filter regeneration system forexecuting a regeneration process of a particulate filter placed in anexhaust passage of an internal combustion engine, wherein the filterregeneration system comprises: an accumulation amount estimation unitfor estimating an accumulation amount of particulate matter at theparticulate filter; a pressure determination unit for determining apressure upstream of the particulate filter in the exhaust passage or adifferential pressure across the particulate filter; and an executionunit for the regeneration process for executing the regeneration processto oxidize and remove the particulate matter accumulated at theparticulate filter; and initiates execution of the regeneration processby the execution unit for the regeneration process, in case theaccumulation amount of the particulate matter estimated by theaccumulation amount estimation unit reaches a pre-determinedregeneration requiring accumulation amount; or in case the pressure orthe differential pressure determined by the pressure determination unitreaches a pre-determined regeneration requiring value, the value beinglarger than the pressure or the differential pressure corresponding tothe regeneration requiring accumulation amount, wherein the abnormalitydiagnosis system for the filter regeneration system comprises: aparameter calculation unit for calculating a ratio of: an accumulationamount of the particulate matter at the initiation of the execution ofthe regeneration process estimated by the accumulation amount estimationunit to the regeneration requiring accumulation amount; as a parameterfor execution frequency of regeneration; and a diagnosis unit fordiagnosing based on the parameter for execution frequency ofregeneration whether or not abnormality is occurring in the filterregeneration system causing excessive execution frequency of theregeneration process.
 2. The abnormality diagnosis system for a filterregeneration system according to claim 1, wherein the diagnosis unitdiagnoses an occurrence of abnormality in the filter regeneration systemcausing excessive execution frequency of the regeneration process, incase the parameter for execution frequency of regeneration Freq issmaller than a predetermined criterion value, wherein the parameter forexecution frequency of regeneration Freq is determined byFreq=Mreq/Mnrm, where Mreq stands for an accumulation amount of theparticulate matter at the initiation of execution of the regenerationprocess estimated by the accumulation amount estimation unit, and Mnrmstands for the regeneration requiring accumulation amount.
 3. Theabnormality diagnosis system for a filter regeneration system accordingto claim 2, further comprising an average calculation unit forcalculating an average value of the parameter for execution frequency ofregeneration when the regeneration process is executed a pre-determinednumber of times; wherein the diagnosis unit diagnoses whether or notabnormality occur in the filter regeneration system causing excessiveexecution frequency of the regeneration process, based on the averagevalue of the parameter for execution frequency of regenerationcalculated by the average calculation unit.
 4. The abnormality diagnosissystem for a filter regeneration system according to claim 3, whereinthe pre-determined number of times is a number of the times ofexecutions of the regeneration process from a previous diagnosis of anoccurrence of abnormality in the filter regeneration system until atravel distance of a vehicle equipped with an internal combustion enginereaches a pre-determined travel distance.
 5. The abnormality diagnosissystem for a filter regeneration system according to claim 1, furthercomprising an average calculation unit for calculating an average valueof the parameter for execution frequency of regeneration when theregeneration process is executed a pre-determined number of times;wherein the diagnosis unit diagnoses whether or not abnormality occur inthe filter regeneration system causing excessive execution frequency ofthe regeneration process, based on the average value of the parameterfor execution frequency of regeneration calculated by the averagecalculation unit.
 6. The abnormality diagnosis system for a filterregeneration system according to claim 5, wherein the pre-determinednumber of times is a number of the times of executions of theregeneration process from a previous diagnosis of an occurrence ofabnormality in the filter regeneration system until a travel distance ofa vehicle equipped with an internal combustion engine reaches apre-determined travel distance.
 7. An abnormality diagnosis system for afilter regeneration system for executing a regeneration process of aparticulate filter placed in an exhaust passage of an internalcombustion engine, wherein the filter regeneration system comprises: anaccumulation amount estimation unit for estimating an accumulationamount of particulate matter at the particulate filter; a pressuredetermination unit for determining a pressure upstream of theparticulate filter in the exhaust passage or a differential pressureacross the particulate filter; and an execution unit for theregeneration process for executing the regeneration process to oxidizeand remove the particulate matter accumulated at the particulate filter;initiates execution of the regeneration process by the execution unitfor the regeneration process, in case the accumulation amount of theparticulate matter estimated by the accumulation amount estimation unitreaches a pre-determined regeneration requiring accumulation amount; orin case the pressure or the differential pressure determined by thepressure determination unit reaches a pre-determined regenerationrequiring value, the value being larger than the pressure or thedifferential pressure corresponding to the regeneration requiringaccumulation amount; and terminates the execution of the regenerationprocess, in case, after the initiation of the execution of theregeneration process, the accumulation amount of the particulate matterat the particulate filter is judged to have decreased to apre-determined base accumulation amount, wherein the abnormalitydiagnosis system for the filter regeneration system comprises: aparameter calculation unit for calculating a ratio of: the differencebetween an accumulation amount of the particulate matter at theinitiation of the execution of the regeneration process estimated by theaccumulation amount estimation unit and the base accumulation amount tothe difference between the regeneration requiring accumulation amountand the base accumulation amount; as a parameter for execution frequencyof regeneration; and a diagnosis unit for diagnosing based on theparameter for execution frequency of regeneration whether or notabnorinality is occurring in the filter regeneration system causingexcessive execution frequency of the regeneration process.
 8. Theabnormality diagnosis system for a filter regeneration system accordingto claim 7, wherein the diagnosis unit diagnoses an occurrence ofabnormality in the filter regeneration system causing excessiveexecution frequency of the regeneration process, in case the parameterfor execution frequency of regeneration Freq′ is smaller than apre-determined criterion value, wherein the parameter for executionfrequency of regeneration Freq′ is determined by Freq′(Mreq−Mbase)/(Mnrm−Mbase), where Mreq stands for an accumulation amountof the particulate matter at the initiation of execution of theregeneration process estimated by the accumulation amount estimationunit, Mnrm stands for the regeneration requiring accumulation amount,and Mbase stands for the base accumulation amount.
 9. The abnormalitydiagnosis system for a filter regeneration system according to claim 8,further comprising an average calculation unit for calculating anaverage value of the parameter for execution frequency of regenerationwhen the regeneration process is executed a pre-determined number oftimes; wherein the diagnosis unit diagnoses whether or not abnormalityoccur in the filter regeneration system causing excessive executionfrequency of the regeneration process, based on the average value of theparameter for execution frequency of regeneration calculated by theaverage calculation unit.
 10. The abnormality diagnosis system for afilter regeneration system according to claim 9, wherein thepre-determined number of times is a number of the times of executions ofthe regeneration process from a previous diagnosis of an occurrence ofabnormality in the filter regeneration system until a travel distance ofa vehicle equipped with an internal combustion engine reaches apre-determined travel distance.
 11. The abnormality diagnosis system fora filter regeneration system according to claim 7, further comprising anaverage calculation unit for calculating an average value of theparameter for execution frequency of regeneration when the regenerationprocess is executed a pre-determined number of times; wherein thediagnosis unit diagnoses whether or not abnormality occur in the filterregeneration system causing excessive execution frequency of theregeneration process, based on the average value of the parameter forexecution frequency of regeneration calculated by the averagecalculation unit.
 12. The abnormality diagnosis system for a filterregeneration system according to claim 11, wherein the pre-determinednumber of times is a number of the times of executions of theregeneration process from a previous diagnosis of an occurrence ofabnormality in the filter regeneration system until a travel distance ofa vehicle equipped with an internal combustion engine reaches apre-determined travel distance.
 13. A method for diagnosing abnormalityin a filter regeneration system, wherein the filter regeneration systemexecutes a regeneration process of a particulate filter placed in anexhaust passage of an internal combustion engine; and comprises anaccumulation amount estimation unit for estimating an accumulationamount of particulate matter at the particulate filter; a pressuredetermination unit for determining a pressure upstream of theparticulate filter in the exhaust passage or a differential pressureacross the particulate filter; and an execution unit for theregeneration process for executing the regeneration process to oxidizeand remove the particulate matter accumulated at the particulate filter;and initiates execution of the regeneration process by the executionunit for the regeneration process, in case the accumulation amount ofthe particulate matter estimated by the accumulation amount estimationunit reaches a pre-determined regeneration requiring accumulationamount; or in case the pressure or the differential pressure determinedby the pressure determination unit reaches a pre-determined regenerationrequiring value, the value being larger than the pressure or thedifferential pressure corresponding to the regeneration requiringaccumulation amount, wherein the method for diagnosing abnormality inthe filter regeneration system comprises: a step for calculating a ratioof an accumulation amount of the particulate matter at the initiation ofthe execution of the regeneration process estimated by the accumulationamount estimation unit to the regeneration requiring accumulationamount, as a parameter for execution frequency of regeneration; and astep for diagnosing based on the parameter for execution frequency ofregeneration whether or not abnormality is occurring in the filterregeneration system causing excessive execution frequency of theregeneration process.
 14. A method for diagnosing abnormality in afilter regeneration system, wherein the filter regeneration systemexecutes a regeneration process of a particulate filter placed in anexhaust passage of an internal combustion engine; and comprises: anaccumulation amount estimation unit for estimating an accumulationamount of particulate matter at the particulate filter; a pressuredetermination unit for determining a pressure upstream of theparticulate filter in the exhaust passage or a differential pressureacross the particulate filter; and an execution unit for theregeneration process for executing the regeneration process to oxidizeand remove the particulate matter accumulated at the particulate filter;and initiates execution of the regeneration process by the executionunit for the regeneration process, in case the accumulation amount ofthe particulate matter estimated by the accumulation amount estimationunit reaches a pre-determined regeneration requiring accumulationamount; or in case the pressure or the differential pressure determinedby the pressure determination unit reaches a pre-determined regenerationrequiring value, the value being larger than the pressure or thedifferential pressure corresponding to the regeneration requiringaccumulation amount; and terminates the execution of the regenerationprocess, in case, after the initiation of the execution of theregeneration process, the accumulation amount of the particulate matterat the particulate filter is judged to have decreased to apre-determined base accumulation amount, wherein the method fordiagnosing abnormality in the filter regeneration system comprises: astep for calculating a ratio of the difference between an accumulationamount of the particulate matter at the initiation of the execution ofthe regeneration process estimated by the accumulation amount estimationunit and the base accumulation amount to the difference between theregeneration requiring accumulation amount and the base accumulationamount, as a parameter for execution frequency of regeneration; and astep for diagnosing based on the parameter for execution frequency ofregeneration whether or not abnormality is occurring in the filterregeneration system causing excessive execution frequency of theregeneration process.